The initial development of photochromic glasses, or phototropic glasses as such have been variously termed, was disclosed in U.S. Pat. No. 3,208,860. The first photochromic glass to enjoy substantial commercial success was utilized in prescription ophthalmic lenses marketed under the trademark PHOTOGRAY.RTM.. That glass was encompassed within the disclosure of U.S. Pat. No. 3,208,860 and had the approximate analysis recorded below in weight percent:
______________________________________ SiO.sub.2 55.6 PbO 5.0 B.sub.2 O.sub.3 16.4 ZrO.sub.2 2.2 Al.sub.2 O.sub.3 8.9 Ag 0.16 Li.sub.2 O 2.65 CuO 0.035 Na.sub.2 O 1.85 Cl 0.24 K.sub.2 O 0.01 Br 0.145 BaO 6.7 F 0.19 CaO 0.2 ______________________________________
Photochromic performance, i.e., the degree of darkening and rate of fading exhibited by a glass, has always been of prime significance in adjudging the utility thereof. Whereas it was recognized that photochromic glasses were activated by radiations in the ultraviolet and low visible portions of the spectrum, laboratory experience indicated that frequently the correlation between data obtained upon exposure to an ultraviolet lamp and data obtained upon exposure to sunlight was quite poor. Consequently, in order to achieve better correlation between indoor laboratory testing and outdoor exposure to sunlight, the solar simulator apparatus described in U.S. Pat. No. 4,125,775 was designed. That apparatus emits radiation which closely approximates the spectral output of the solar spectrum.
The extensive research which has been expended in the development of photochromic glasses has revealed that the dynamics of photochromic behavior are dependent to a greater or lesser extent upon the ambient temperature. Accordingly, where other factors are held constant, a photochromic glass will generally darken to a lower transmission when exposed to actinic radiation at a low temperature, e.g., 0.degree. C. (32.degree. F.), than at a temperature of, say 40.degree. C. (104.degree. F.). Furthermore, photochromic glasses typically fade more slowly at low temperatures when the actinic radiation is removed than at higher temperatures. This variance in behavior has been defined as the "temperature dependence" of a glass. As can be appreciated, except for an unusual application, the lower the temperature dependence exhibited by a glass, the better suited it will be for general utility.
The initial research in ophthalmic photochromic lenses, exemplified by the above-described PHOTOGRAY.RTM. lenses, produced glasses which darkened only to the "comfort range", i.e., to luminous transmittances in the vicinity of about 40-60%, when subjected to actinic radiation, and faded quite slowly when removed from the actinic radiation. Consumer demand, however, fostered further research to develop glasses which could function as sunglasses outdoors, i.e., darken to luminous transmittances below 40% and desirably considerably less, and which would fade rapidly when worn indoors. That demand resulted in the production of prescription lenses marketed under the trademark PHOTOGRAY EXTRA.RTM. which darken to a considerably lower luminous transmittance and fade much more rapidly than PHOTOGRAY.RTM. lenses. The glass for PHOTOGRAY EXTRA lenses is included within the composition ranges described in U.S. Pat. No. 4,190,451. That patent discloses glasses which, in 2 mm cross section, exhibit the following photochromic characteristics:
(a) at about 20.degree. C. the glasses will darken to a luminous transmittance below 40% in the presence of actinic radiation; will fade at least 30 percentage units of transmittance after five minutes' removal from the actinic radiation; and will facde to a luminous transmittance greater than 80% within two hours after being removed from the actinic radiation; PA0 (b) at about 40.degree. C. the glasses will darken to a luminous transmittance below 55% in the presence of actinic radiation; will fade at least 25 percentage units of transmittance after five minutes' removal from the actinic radiation; and will fade to a luminous transmittance in excess of 80% within two hours after being removed from the actinic radiation; and PA0 (c) at about -18.degree. C. the glasses will not darken to a luminous transmittance below 15% in the presence of actinic radiation. PA0 (1) in a thickness of about 2 mm the glasses will demonstrate the following photochromic properties: PA0 (2) the glasses are capable of being strengthened through thermal tempering or chemical strengthening while essentially retaining the desired photochromic properties; and PA0 (3) the glasses have compositions suitable for refractive index adjustment without substantial loss of the desired photochromic properties.
The glasses of U.S. Pat. No. 4,190,451 consist essentially, expressed in terms of weight percent on the oxide basis, of:
______________________________________ Li.sub.2 O 0-2.5 P.sub.2 O.sub.5 0-25 Na.sub.2 O 0-9 SiO.sub.2 20-65 K.sub.2 O 0-17 CuO 0.004-0.02 Cs.sub.2 O 0-6 Ag 0.15-0.3 Li.sub.2 O + Na.sub.2 O + K.sub.2 O + Cs.sub.2 O 8-20 Cl 0.1-0.25 B.sub.2 O.sub.3 14-23 Br 0.1-0.2 Al.sub.2 O.sub.3 5-25 molar ratio alkali metal oxide: B.sub.2 O.sub.3 0.55-0.85 weight ratio Ag: (Cl + Br) 0.65-0.95 ______________________________________
Another evidence of research to develop glasses demonstrating improved photochromic performance is found in U.S. Pat. No. 4,102,693. Those glasses are stated to have a half fading time of not more than 60 seconds and contain silver halide crystals to provide photochromism. The base glass compositions are free from BaO and consist essentially, in weight percent, of:
______________________________________ SiO.sub.2 31-59 P.sub.2 O.sub.5 0-12 B.sub.2 O.sub.3 18-28 ZrO.sub.2 0-7 Al.sub.2 O.sub.3 8-20 TiO.sub.2 0-5 Li.sub.2 O 0-3 PbO 0-7 Na.sub.2 O 0-8 Ag.sub.2 O 0.05-4 K.sub.2 O 0-16 F 0-0.2 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 6-16 Cl 0.04-0.5 MgO 0-2.6 Br 0-1 CuO 0-1 F + Cl + Br 0.13-1 ______________________________________
A more recent advance in providing photochromic glasses exhibiting very fast fading capabilities, coupled with a high degree of darkening, is described in U.S. Pat. No. 4,407,966. The glasses disclosed therein are asserted to darken to a luminous transmittance below 35% at a temperature of 40.degree. C. and will fade at least 40 percentage units of transmittance after five minutes' removal from the actinic radiation; will darken to a luminous transmittance below 25% at a temperature of 25.degree. C. and will fade at least 35 percentage units of transmittance after five minutes' removal from the actinic radiation; and will retain a darkened luminous transmittance greater than 15% at 0.degree. C. The glasses consist essentially, in weight percent, of:
______________________________________ SiO.sub.2 56-70 PbO 0.1-0.15 Al.sub.2 O.sub.3 6-9 Ag &gt;0.15-0.25 B.sub.2 O.sub.3 18-21 Cl 0.2-0.35 Li.sub.2 O &gt;2.5-3.5 Br 0.075-0.15 Na.sub.2 O 0.5-2.5 CuO 0.004-0.02 K.sub.2 O 5-7 CeO.sub.2 0-0.5 ZrO.sub.2 3.75-5 ______________________________________
Whereas the principal thrust of the research with respect to photochromic glass has been directed toward developing compositions which would darken to low luminous transmittance values, i.e., compositions suitable for use as sunglasses, and fade ever more rapidly, there has been a continuing demand for glasses which would darken only to the "comfort range", but which would fade much more rapidly than the PHOTOGRAY.RTM. lenses described above. Hence, as is illustrated in U.S. Pat. No. 4,190,451, PHOTOGRAY.RTM. lenses darken to a luminous transmittance of 53.5% at 40.degree. C. and fade 23 percentage units of transmittance after five minutes' removal from the actinic radiation; and darken to a luminous transmittance of 37.5% at 0.degree. C. and fade 10 percentage units of transmittance after five minutes' removal from the actinic radiation.
Market research data had disclosed a desire for photochromic ophthalmic lenses which would exhibit good temperature dependence over the ambient temperature range of about 0.degree.-40.degree. C. (32.degree.-104.degree. F.) and would darken to a luminous transmittance between about 40-60%, preferably about 45-50%, within that ambient temperature range. As defined herein, good temperature dependence signifies a difference in darkened luminous transmittance of less than 15 percentage units of transmittance, preferably less than 10, over the range of 0.degree.-40.degree. C. The data also indicated that the lenses should fade at least 30 percentage units of transmittance after five minutes' removal from the actinic radiation.
In view of previous experience, it was recognized that the rate of fading at cooler temperatures becomes quite slow. That phenomenon, however, has not been adjudged to be of substantial practical significance because, more often than not, the circumstances will be such that the glass is warming up while it is fading. For example, the wearer of the eyeglasses will be coming indoors from being outdoors on a cold day such that the rise in temperature contacting the lenses will increase their fade rate.
Accordingly, the following criteria for the inventive glasses were devised:
(a) over the range of about 0.degree.-40.degree. C. the glasses will darken to a luminous transmittance in the presence of actinic radiation, e.g., outdoor sunlight, of between about 40-60%, preferably about 45-50%; PA1 (b) over the range of about 0.degree.-40.degree. C. the difference in darkened luminous transmittance exhibited by the glasses will be less than 15 percentage units of transmittance, preferably less than 10; and PA1 (c) over the range of about 25.degree.-40.degree. C. the glasses will fade at least 30 percentage units of transmittance after five minutes' removal from the actinic radiation;
As employed herein, the luminous transmittance of a glass is defined as the value Y delineated in terms of the 1931 C.I.E. trichromatic colorimetric system utilizing the light source Illuminant C. This colorimetric system and light source are described by A. C. Hardy in the Handbook of Colorimetry, Technology Press, M.I.T., Cambridge, Mass. (1936).